1. Field of the Invention
The present invention relates to an inductive load drive circuit and, more particularly, to an inductive load drive circuit which prevents back electromotive force from flowing into the load power source.
2. Description of the Related Art
Electric motors, which operate on battery power, are used in cameras to wind camera film, drive the camera lens and act as other types of actuators. An electric motor used in a camera is typically driven by a bridge circuit type driver IC comprising various transistors. The motor achieves the operation modes of normal rotation (flow in a normal direction), reverse rotation (flow in a reverse direction), braking (grounding and shorting both terminals of the motor) and stopping (both terminals of the motor are open) by various ON and OFF combinations of the bridge circuit transistors.
In order to prevent breakdowns in the bridge circuit transistors and other components due to surge current when both terminals of the motor are open, surge killer diodes are connected in parallel in a direction opposite to the normal electric current direction of each transistor in the bridge circuit.
When each transistor of the bridge circuit is cut off (when the bridge circuit is open in both directions with respect to the motor) and the motor is rotating by inertia, back electromotive force of the motor is generated. The back electromotive force produces an electric current which flows through the surge killer diodes into the battery power source through a battery terminal. In effect, a charging condition is produced through the surge killer diodes of the bridge circuit.
This charging condition is undesirable for a power source battery that cannot be charged. The charging condition can cause the battery to heat up or develop fluid leaks.